BILL O'REILLY, HOST: For the top story tonight we are pleased to bring to you from Washington the Secretary of Homeland Security, Michael Chertoff.

Well, I know you're exhausted. You've been running around. And we really appreciate you coming on "The Factor."

In this whole process, what was the most startling thing you learned, sir?

MICHAEL CHERTOFF, HOMELAND SECURITY SECRETARY: You know, I think for me it was discovering the breadth of this plot. Unlike last year's London bombing, which wound up being four people and then a follow-on bombing where attempted with four others, we're talking about over 20 people here. And that's without even finishing all of the investigations.

So the scope of the plot, the sophistication, the careful design of the bomb detonation mechanisms, all of this really brought me back, frankly, to 9/11, which is the precedent, I think, about for a very strategic, very capable and very dangerous kind of top-grade terrorist action.

O'REILLY: All right. Now we understand that the liquid was some combustible mixture, which could be ignited by, say, an iPod, or some kind of other electrical device, maybe a cell phone.

Now I don't expect you to tell me what it was. This is security and all of that. But if liquid is now a deadly weapon, as I said in the "Talking Points Memo," I mean, you're not going to be able to have a contact lens solution case on you. You're not going to be able to have some water that you carry on the plane. Is that the future of the air travel?

CHERTOFF: Well, I tell you the problem here was not only that it's liquid, because obviously liquids that are exposed sometimes have a very distinctive smell or there may be other characteristics that are recognizable.

The challenge here was that they were designing a device that would really conceal the nature of the liquid by making it seem like it was ordinary. Let's say an ordinary beverage or something like that in an ordinary container. Unlike what we think of, which is a typical bomb with wires coming out or a detonator that's strapped onto the explosive powder or the explosive liquid.

Here, I think we were facing the possibility of someone bringing in individual components, each of which would look very innocuous and very common, and then literally assembling the bomb in the final minutes on the plane itself.

O'REILLY: But the solution to that is not to allow anybody with any liquid or any carry-on luggage? I mean, I'm looking for how you stop this kind of a thing. It was great investigative work. Pakistanis, British, U.S., all cooperating. Our NSA intercepting phone calls, I understand. But for you and me and all the other Americans, and we have to travel, and we're not going to be able to carry anything on the plane any more?

CHERTOFF: Well, here's what we have to do. We have to get the device. We have to reverse engineer it. And once we reverse engineer it, once we actually see what they constructed, we can then take our screening tools, our various kinds of detection equipment, our x-rays, and we can see about recalibrating it to detect those characteristics of this type of device...

O'REILLY: Got it, got it.

CHERTOFF: ...that make it distinct.

O'REILLY: OK. So you're not going make any judgments until you do that kind of analysis, and then figure out...

CHERTOFF: That's right.

O'REILLY: ...how to combat it.

CHERTOFF: But we're going to be safe, but while doing that, we're going to be safe rather than — sorry, Bill. And that's why we are putting this measure in effect about screening all liquids out, so we have the time to do the work we need to do.

O'REILLY: OK, but people still can carry on luggage. They're just not allowed to take liquids on.

CHERTOFF: That's right. We are not banning carry-on luggage, only liquids.

O'REILLY: OK. Code red. This is the first time we're in a code red situation here in the USA. Why are we in that? Because there may be more of these guys running around?

CHERTOFF: Well, we think the British succeeded in disrupting the plot. And there's no question the focal point of the operational activity was in Britain.

But their judgment was that given the haste with which they had to move, they couldn't be 100 percent sure they had scooped up everybody. Nor were they sure there weren't other affiliated parts of the network still at large and still capable of carrying out a bombing.

So their judgment was — the really safe course is to put the highest level of alert in.

Now we really took a lot of our cue from them because they had the boots on ground. They had the firsthand knowledge. And when they made the decision to go up to their equivalent of red, it was a pretty easy judgment after that to conclude it would be sensible for us to us match them with respect to that particular set of flights.

O'REILLY: OK. Last question for you. The fact that the NSA was able to intercept some of these phone calls that were made in the United States to Al Qaeda in Britain by using the very controversial — although I understand warrants were obtained for this by the FISA court. In your opinion, does that mean that the Bush administration is justified now in its original policy? Is this a big win politically for you guys?

CHERTOFF: Well, Bill, of course I'm not going to confirm particular techniques were used, but I do think this.

O'REILLY: You won't deny, though.

CHERTOFF: Obviously I'm not going to discuss classified techniques.

But let me address the point, which is I think this is a very sobering moment for the American people. Having been in the cockpit, so to speak, in the last few days, trying to put this thing together, work, you know, with my colleagues and other agencies to see what the threat was in the U.S., you know you're dealing with a very formidable enemy and one who is ruthless and will do anything. You don't want to go into that battle leaving your weapons on the table.

You can't afford to fight that battle if you don't use every single legal tool to the full extent that you're entitled to do.

O'REILLY: All right. Mr. Secretary, pleasure to have you on the program this evening. Thank you very much.

CHERTOFF: You're welcome. Good to be on.

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